1. Field of the Invention
Provided is the preparation and use of novel transition metal catalysts, which can be used to generate olefins from alkyl-containing molecules. More specifically, novel iridium metal catalysts with benzimidazolyl-containing ligands coordinated with the iridium are provided, which are useful for generating olefins from alkanes.
2. Description of the Related Art
Olefins can be generated by direct dehydrogenation with the removal of hydrogen gas or by the use of an acceptor such as ethylene to generate ethane. The chemical industry uses olefins as intermediates in a variety of processes. The largest chemical use is linear α-olefins used in the formation of polyolefins such as ethylene-1-octene copolymers. Also and most importantly, low carbon number olefins have the potential to be converted into higher carbon number molecules that would be suitable for fuels, particularly, diesel. Other products formed from olefins include surfactants, lubricants, and plasticizers.
Iridium complexes as catalysts are known. During the 1980s, it was discovered that certain iridium complexes are capable of catalytically dehydrogenating alkanes to alkenes under exceptionally mild thermal (i.e., less than 160° C.) or even photolytic conditions (see, e.g., J. Am. Chem. Soc. 104 (1982) 107; 109 (1987) 8025; J. Chem. Soc., Chem. Commun. (1985) 1829). For a more recent example, see Organometallics 15 (1996) 1532.
Pincer ligand complexes of rhodium and iridium as catalysts for the dehydrogenation of alkanes are receiving widespread attention. See, for example, F. Liu, E. Pak, B. Singh, C. M. Jensen and A. S. Goldman, “Dehydrogenation of n-Alkanes Catalyzed by Iridium “Pincer” Complexes: Regioselective Formation of alpha-olefins,” J. Am. Chem. Soc. 121 (1999) 4086-4087; F. Liu and A. S. Goldman, “Efficient thermochemical alkane dehydrogenation and isomerization catalyzed by an iridium pincer complex,” Chem. Comm. 1999, 655-656; and C. M. Jensen, “Iridium PCP pincer complexes: highly active and robust catalysts for novel homogenous aliphatic dehydrogenations,” Chem. Comm. (1999) 2443-2449. The use of compounds such as (PCP)MH2 (PCP=C6H3(CH2PBut2)2-2,6) (M=Rh, Ir) (1a, 1b) dehydrogenate various cycloalkanes to cycloalkenes at 200° C. with turnovers of 70-80 turnovers/hour. The reaction proceeds at 200° C. in neat solvent and without the use of a sacrificial hydrogen acceptor such as tert-butyl ethylene.
In addition, “pincer” complexes of platinum-group metals have been known since the late 1970s (see, e.g., J. Chem. Soc., Dalton Trans. (1976) 1020). Pincer complexes have a metal center and a pincer skeleton. The pincer skeleton is a tridentate ligand that generally coordinates with the meridional geometry. The use of pincer complexes in organic synthesis, including their use as low-temperature alkane dehydrogenation catalysts, was exploited during the 1990s and is the subject of two review articles (see Angew. Chem. Int. Ed 40 (2001) 3751 and Tetrahedron 59 (2003) 1837). See also U.S. Pat. No. 5,780,701. Jensen et al. (Chem. Commun. (1997) 461) used iridium pincer complexes to dehydrogenate ethylbenzene to styrene at 150 to 200° C. Recently, pincer complexes have been developed that dehydrogenate hydrocarbons at even lower temperatures. For some recent examples, see J. Mol. Catal. A 189 (2002) 95, 111 and Chem. Commun. (1999) 2443.
Despite the extensive research into new catalysts and methods for producing valuable olefin compounds, the search for catalysts which are more stable, as well as more thermally and chemically robust continues. Such catalysts would make the preparation of valuable olefin compounds more economic and efficient.